X-ray emission spectroscopy (XES) is a form of
X-ray spectroscopy
X-ray spectroscopy is a general term for several spectroscopic techniques for characterization of materials by using x-ray radiation.
Characteristic X-ray spectroscopy
When an electron from the inner shell of an atom is excited by the energy o ...
in which the
X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
line
spectra are measured with a
spectral resolution The spectral resolution of a spectrograph, or, more generally, of a frequency spectrum, is a measure of its ability to resolve features in the electromagnetic spectrum. It is usually denoted by \Delta\lambda, and is closely related to the resolvi ...
sufficient to analyze the impact of the chemical environment on the X-ray line energy and on branching ratios.
This is done by exciting
electrons
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
out of their
shell
Shell may refer to:
Architecture and design
* Shell (structure), a thin structure
** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses
** Thin-shell structure
Science Biology
* Seashell, a hard o ...
and then watching the emitted
photons
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alway ...
of the recombinating electrons.
There are several types of XES and can be categorized as non-resonant XES (XES), which includes
-measurements, valence-to-core (VtC/V2C)-measurements, and (
)-measurements, or as resonant XES (RXES or
RIXS
Resonant inelastic X-ray scattering (RIXS) is an X-ray spectroscopy technique used to investigate the electronic structure of molecules and materials.
Inelastic X-ray scattering is a fast developing experimental technique in which one scatters hi ...
), which includes XXAS+XES 2D-measurement, high-resolution
XAS
X-ray absorption spectroscopy (XAS) is a widely used technique for determining the local geometric and/or electronic structure of matter. The experiment is usually performed at synchrotron radiation facilities, which provide intense and tunabl ...
, 2p3d RIXS, and
Mössbauer-XES-combined measurements.
[S. DeBeer]
''Advanced X-Ray Spectroscopy''
(PDF) Juni 2016, last checked 26.02.2020 In addition, ''Soft X-ray emission spectroscopy'' (SXES) is used in determining the
electronic structure
In quantum chemistry, electronic structure is the state of motion of electrons in an electrostatic field created by stationary nuclei. The term encompasses both the wave functions of the electrons and the energies associated with them. Electro ...
of materials.
History
The first XES experiments were published by Lindh and Lundquist in 1924
![Energy Level Diagramm K-Lines](https://upload.wikimedia.org/wikipedia/commons/a/a2/Energy_Level_Diagramm_K-Lines.jpeg)
In these early studies, the authors utilized the electron beam of an X-ray tube to excite core electrons and obtain the
-line spectra of
sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
and other
elements. Three years later, Coster and Druyvesteyn performed the first experiments using photon excitation. Their work demonstrated that the electron beams produce
artifacts, thus motivating the use of X-ray photons for creating the core hole. Subsequent experiments were carried out with commercial X-ray spectrometers, as well as with high-resolution spectrometers.
While these early studies provided fundamental insights into the electronic configuration of small molecules, XES only came into broader use with the availability of high intensity X-ray beams at
synchrotron radiation
Synchrotron radiation (also known as magnetobremsstrahlung radiation) is the electromagnetic radiation emitted when relativistic charged particles are subject to an acceleration perpendicular to their velocity (). It is produced artificially in ...
facilities, which enabled the measurement of (chemically) dilute samples.
In addition to the experimental advances, it is also the progress in quantum chemical computations, which makes XES an intriguing tool for the study of the electronic structure of
chemical compounds
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
.
Henry Moseley
Henry Gwyn Jeffreys Moseley (; 23 November 1887 – 10 August 1915) was an English physicist, whose contribution to the science of physics was the justification from physical laws of the previous empirical and chemical concept of the atomic num ...
, a British physicist was the first to discover a relation between the
-lines and the atomic numbers of the probed elements. This was the birth hour of modern x-ray spectroscopy.
Later these lines could be used in elemental analysis to determine the contents of a sample.
William Lawrence Bragg
Sir William Lawrence Bragg, (31 March 1890 – 1 July 1971) was an Australian-born British physicist and X-ray crystallographer, discoverer (1912) of Bragg's law of X-ray diffraction, which is basic for the determination of crystal structu ...
later found a relation between the energy of a photon and its diffraction within a crystal. The formula he established,
says that an X-ray photon with a certain energy bends at a precisely defined angle within a crystal.
Equipment
Analyzers
A special kind of
monochromator
A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input. The name is from the Greek roots ''mono-'', "s ...
is needed to diffract the radiation produced in X-Ray-Sources. This is because X-rays have a refractive index ''n ≈ 1''. Bragg came up with the
equation
In mathematics, an equation is a formula that expresses the equality of two expressions, by connecting them with the equals sign . The word ''equation'' and its cognates in other languages may have subtly different meanings; for example, in ...
that describes x-ray/
neutron
The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...
diffraction
Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
when those particles pass a crystal lattice.(
X-ray diffraction
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
)
For this purpose "
perfect crystal
Crystalline materials (mainly metals and alloys, but also stoichiometric salts and other materials) are made up of solid regions of ordered matter (atoms placed in one of a number of ordered formations called Bravais lattices). These regions are kn ...
s" have been produced in many shapes, depending on the geometry and energy range of the instrument. Although they are called perfect, there are miscuts within the crystal structure which leads to offsets of the
Rowland plane.
These offsets can be corrected by turning the crystal while looking at a specific energy(for example:
-line of copper at 8027.83eV).
When the intensity of the signal is maximized, the photons diffracted by the crystal hit the detector in the Rowland plane.
There will now be a slight offset in the horizontal plane of the instrument which can be corrected by increasing or decreasing the detector angle.
In the Von Hamos geometry, a cylindrically bent crystal disperses the radiation along its flat surface's plane and focuses it along its axis of curvature onto a line like feature.
![Rowland Circle with two orders](https://upload.wikimedia.org/wikipedia/commons/3/3f/Rowland_Circle_with_two_orders.png)
The spatially distributed signal is recorded with a position sensitive detector at the crystal's focusing axis providing the overall spectrum.
Alternative wavelength dispersive concepts have been proposed and implemented based on Johansson geometry having the source positioned inside
the Rowland circle, whereas an instrument based on Johann geometry has its source placed on the Rowland circle.
X-ray sources
X-ray sources are produced for many different purposes, yet not every X-ray source can be used for spectroscopy. Commonly used sources for medical applications generally generate very "noisy" source spectra, because the used cathode material must not be very pure for these measurements. These lines must be eliminated as much as possible to get a good resolution in all used energy ranges.
For this purpose normal X-ray tubes with highly pure tungsten, molybdenum, palladium, etc. are made. Except for the copper they are embedded in, the produce a relatively "white" spectrum.
Another way of producing X-rays are particle accelerators. The way they produce X-rays is from vectorial changes of their direction through magnetic fields. Every time a moving charge changes direction it has to give off radiation of corresponding energy. In X-ray tubes this directional change is the electron hitting the metal target (Anode) in synchrotrons it is the outer magnetic field accelerating the electron into a circular path.
There are many different kind of X-ray tubes and operators have to choose accurately depending on what it is, that should be measured.
Modern spectroscopy and the importance of -lines in the 21st Century
Today, XES is less used for elemental analysis, but more and more do measurements of
-line spectra find importance,
as the relation between these lines and the
electronic structure
In quantum chemistry, electronic structure is the state of motion of electrons in an electrostatic field created by stationary nuclei. The term encompasses both the wave functions of the electrons and the energies associated with them. Electro ...
of the ionized atom becomes more detailed.
If an 1s-Core-Electron gets excited into the continuum(out of the atoms energy levels in MO),
electrons of higher energy orbitals need to lose energy and "fall" to the 1s-Hole that was created to fulfill Hund's Rule.(Fig.2)
Those electron transfers happen with distinct probabilities. (See
Siegbahn notation The Siegbahn notation is used in X-ray spectroscopy to name the spectral lines that are characteristic to elements. It was introduced by Manne Siegbahn.
The characteristic lines in X-ray emission spectra correspond to atomic electronic transiti ...
)
Scientists noted that after an ionisation of a somehow bonded 3d-transition metal-atom the
-lines intensities and energies shift
with oxidation state of the metal and with the species of ligand(s). This gave way to a new method in structural analysis:
By high-resolution scans of these lines the exact energy level and structural configuration of a chemical compound can be determined.
This is because there are only two major electron transfer mechanisms, if we ignore every transfer not affecting valence electrons.
If we include the fact that chemical compounds of 3d-transition metals can either be high-spin or low-spin we get 2 mechanisms for each spin configuration.
These two spin configurations determine the general shape of the
and
-mainlines as seen in figure one and two,
while the structural configuration of electrons within the compound causes different intensities, broadening, tailing and piloting of the
and
-lines.
Although this is quite a lot of information, this data has to be combined with absorption measurements of the so-called "pre-edge" region.
Those measurements are called XANES (
X-ray absorption near edge structure
X-ray absorption near edge structure (XANES), also known as near edge X-ray absorption fine structure (NEXAFS), is a type of absorption spectroscopy that indicates the features in the X-ray absorption spectra ( XAS) of condensed matter due to the ...
).
![XAS Measurement against HERFD](https://upload.wikimedia.org/wikipedia/commons/2/2a/XAS_Measurement_against_HERFD.png)
In synchrotron facilities those measurement can be done at the same time,
yet the experiment setup is quite complex and needs exact and fine tuned crystal monochromators to diffract the tangential beam coming from the electron storage ring.
Method is called HERFD, which stands for High Energy Resolution Fluorescence Detection.
The collection method is unique in that, after a collection of all wavelengths coming from "the source" called
,
the beam is then shone onto the sampleholder with a detector behind it for the XANES part of the measurement.
The sample itself starts to emit X-rays and after those photons have been monochromatized they are collected, too.
Most setups use at least three crystal monochromators or more.
The
is used in absorption measurements as a part of the
Beer-Lambert Law in the equation
:
where
is the intensity of transmitted photons.
The received values for the extinction
are wavelength specific which therefore creates a spectrum of the absorption.
The spectrum produced from the combined data shows clear advantage in that background radiation is almost completely eliminated while still having an extremely resoluted view on features on a given absorption edge.(Fig.4)
In the field of development of new catalysts for more efficient energy storage, production and usage in form of hydrogen fuelcells and new battery materials,
the research of the
-lines is essential nowadays.
The exact shape of specific oxidation states of metals is mostly known, yet newly produced chemical compounds with the potential
of becoming a reasonable catalyst for electrolysis, for example, are measured every day.
Several countries encourage many different facilities all over the globe in this special field of science in the hope for clean, responsible and cheap energy.
[C. Daller]
''Soft X-ray Emission Spectroscopy at ESRF Beamline 26 Based on a Helical Undulator''
1996, last checked 21.07.2020
Soft x-ray emission spectroscopy
Soft X-ray emission spectroscopy or (SXES) is an experimental technique for determining the
electronic structure
In quantum chemistry, electronic structure is the state of motion of electrons in an electrostatic field created by stationary nuclei. The term encompasses both the wave functions of the electrons and the energies associated with them. Electro ...
of materials.
Uses
X-ray emission spectroscopy (XES) provides a means of probing the partial occupied density of electronic states of a material. XES is
element-specific and site-specific, making it a powerful tool for determining detailed electronic properties of materials.
Forms
Emission spectroscopy can take the form of either resonant inelastic X-ray emission spectroscopy (
RIXS
Resonant inelastic X-ray scattering (RIXS) is an X-ray spectroscopy technique used to investigate the electronic structure of molecules and materials.
Inelastic X-ray scattering is a fast developing experimental technique in which one scatters hi ...
) or non-resonant X-ray emission spectroscopy (
NXES). Both spectroscopies involve the photonic promotion of a core level
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
, and the measurement of the
fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...
that occurs as the electron relaxes into a lower-energy state. The differences between resonant and non-resonant excitation arise from the state of the atom before fluorescence occurs.
In resonant excitation, the core electron is promoted to a
bound state
Bound or bounds may refer to:
Mathematics
* Bound variable
* Upper and lower bounds, observed limits of mathematical functions
Physics
* Bound state, a particle that has a tendency to remain localized in one or more regions of space
Geography
*B ...
in the
conduction band
In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in w ...
. Non-resonant excitation occurs when the incoming radiation promotes a core electron to the continuum. When a core
hole
A hole is an opening in or through a particular medium, usually a solid body. Holes occur through natural and artificial processes, and may be useful for various purposes, or may represent a problem needing to be addressed in many fields of en ...
is created in this way, it is possible for it to be refilled through one of several different decay paths. Because the core hole is refilled from the sample's high-energy free states, the decay and emission processes must be treated as separate
dipole transitions. This is in contrast with
RIXS
Resonant inelastic X-ray scattering (RIXS) is an X-ray spectroscopy technique used to investigate the electronic structure of molecules and materials.
Inelastic X-ray scattering is a fast developing experimental technique in which one scatters hi ...
, where the events are coupled, and must be treated as a single scattering process.
Properties
Soft X-rays have different optical properties than visible light and therefore experiments must take place in
ultra high vacuum
Ultra-high vacuum (UHV) is the vacuum regime characterised by pressures lower than about . UHV conditions are created by pumping the gas out of a UHV chamber. At these low pressures the mean free path of a gas molecule is greater than approximately ...
, where the photon beam is manipulated using special mirrors and
diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure that diffracts light into several beams travelling in different directions (i.e., different diffraction angles). The emerging coloration is a form of structura ...
s.
Gratings diffract each energy or
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
present in the incoming radiation in a different direction. Grating
monochromator
A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input. The name is from the Greek roots ''mono-'', "s ...
s allow the user to select the specific photon energy they wish to use to excite the sample. Diffraction gratings are also used in the
spectrometer
A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the ...
to analyze the photon energy of the radiation emitted by the sample.
See also
*
X-ray absorption spectroscopy
X-ray absorption spectroscopy (XAS) is a widely used technique for determining the local geometric and/or electronic structure of matter. The experiment is usually performed at synchrotron radiation facilities, which provide intense and tunabl ...
References
External links
Soft X-ray Emission Spectroscopy- Description at beamteam.usask.ca
{{Improve categories, date=July 2020
Emission spectroscopy
X-rays
X-ray spectroscopy